DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
JLex Comment - JLex Comment is a comment system for Joomla.
GraphComment - Add modern discussion and social interactions to your website
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.
Thrive Comments - Thrive Comments is a comment system for Wordpress.